Learn how to use the species conversion table (Figure 10.1).
Each species on a site has a unique SI. Site index does not differ by layer, only by species.
It is possible to predict the SI of one species from the SI determined for another species on the same site. Surveyors can use the site index species conversion table to make these predictions.
Conversions may be needed if GI data is collected for a species that is not the inventory component leading species. This could occur if there is no GI table for the inventory species but there is one for the second species on site, or if the leading species has an unacceptable level of damage, disease, or suppression.
The accuracy of the SI value is reduced when the SI species is converted. A SI obtained by converting site index species is more accurate than one obtained from the site class conversion table and, in some instances, better than one obtained with the BEC method. The typical accuracy of the conversion is illustrated in Figure 10.2 which shows the data used to develop the average Ss-Hw site index relationship.
Many, but not all possible, paired species conversions are available to date. A copy of the site index species conversion table is provided in Appendix 5 Average site index relationships between species. A six-ring field book sized version of the species conversion table is available in the FS 415 series. The FS 415 I is the field card for species conversion.
Given that you have surveyed an interior stand with Fd leading species but collected information only on Hw (Fd has leader damage), and you determined the average SI for Hw to be 20 m, what would the SI be for Fd?